Why so many mammalian spermatozoa–a clue from marsupials?
Mammals generally ejaculate many more spermatozoa than seem to be needed for fertilization. This apparent profligacy has not been explained, but observations made in marsupials may shed light on it. The Virginia opossum, Didelphis virginiana, inseminates only about three million spermatozoa, a very low number. As a corollary, relatively few (ca. 13 X 10(6] are stored in each cauda epididymidis. However, some 5% of the spermatozoa that the opossum ejaculates populate the oviduct about 12 h later when ovulation can be anticipated–a success rate in the female orders of magnitude greater than in eutherian mammals. It is not certain what determines the unusually efficient transport to and the high survival rate of spermatozoa in the oviduct of Didelphis, but two unusual features suggest themselves as possible contributors. Didelphis (and all other American marsupial) spermatozoa undergo a head-to-head pairing in the epididymis by the acrosomal face; this serves to isolate the acrosome of e
